Import by url - Move data attributes into helper method in import_helper.rb
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## Summary
Follow up issue to address the comment raised in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/221798#note_3196955314, once merged.
This is a refactor that should have no user-facing impact.
### Implementation Plan
* Create a import_by_url_data_attributes method in app/helpers/import_helper.rb
* Add spec if needed in spec/helpers/import_helper_spec.rb'
* Simplify the view in `app/views/projects/imports/new.html.haml `. It might look like
```
.js-vue-import-by-url-to-project-root{ data: import_by_url_data_attributes(@project, ci_cd_only, git_timeout, @repository_mirrors_available... etc) }
```
For how to view the Import by URL into existing project form, see the instructions in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/221798#how-to-set-up-and-validate-locally . Note that there are two URL import forms. This work item is for just the 'retry' form:
* `app/views/projects/imports/new.html.haml`
* `app/assets/javascripts/projects/new_v2/components/import_by_url_to_existing_project_form.vue`
issue